1. Field of the Invention
The present invention relates to an optical-signal/electric-signal converting device for converting an optical signal into an electric signal and converting an electric signal into an optical signal in a communications system transmitting both an optical signal and an electric signal, that is, a communications system for establishing optical and electrical communications. For example, the invention relates to an optical-signal/electric-signal converting device for use in an industrial measurement and control system including an optical fiber instrumentation system.
2. Description of the related Art
FIG. 1 shows an example of a conventional communications system establishing both optical and electrical communications.
In the conventional communications system, each electric device 6 is connected to an electric master 2 through a transmission line 7 (hereinafter referred also to an electric transmission line). Each optical device 8 is connected to an optical master 3 through an optical transmission line 9 and an optical signal distributor/coupler (for example, an optical star coupler) 5. The electric master 2 and optical master 3 are connected to a host computer 1 which communicates with the electric devices 6 and optical devices 8.
When both optical devices and electric devices are used as in the above described communications system, two different masters of the optical master 3 and electric master 2 are required for the optical device and electric device, respectively. The two types of the masters are respectively used for the following reasons.
(1) Different communications media are required. PA0 (2) Different electric supply systems are used. That is, an electric device is supplied with electric power through a bus electric supply system while an optical device is a battery-driven device. PA0 (3) Different communications protocols are used for the electric device and optical device because the consumption of electric current is limited, in the optical device. Normally, the optical device uses a protocol which requires fewer times of light-emission and also requires a short time light-emission for long battery life. PA0 (1) A system for both optical and electrical communications can be easily configured at a low cost. PA0 (2) Since an optical transmission line can be easily used in a communications system, the transmission line can be readily extended using a longer optical transmission line, even in circumstances in which transmission signals in an electric transmission line are subject to noise, or where an effective explosion-proof method is required for the transmission line. PA0 (3) The abnormal condition of an electric transmission line can be informed to each of the devices in the communications system using a microcomputer. PA0 (4) The communications system for both optical and electrical communications can be easily realized at a low cost even when different protocols are used for the optical and electrical communications.
As described above, in the conventional communications system using both optical and electric signals, a master is required for each medium. Normally, a master is an expensive device provided with, for example, a personal computer, etc. Accordingly, when two masters are required, the system costs a lot. Furthermore, when an optical-communications protocol is incorporated into a general-purpose electrical communications protocol to commonly use the optical and electrical communications protocols, the optical communications time becomes longer and the life-time of the battery for the optical device is shortened.